The present invention relates to a highly ductile bulk metallic glass, cold worked products made of the highly ductile bulk metallic glass, and its production method.
A popular casting method for producing bulk amorphous alloy product is disclosed, for example, in Japanese patent Kokai H5-253656, in which a tubular product is made by forming a cavity of a combination of a core and a mold made of a metal having a high thermal conductivity, and a melt of such as La based alloy or Zr based alloy is injected into the cavity. A xc2xd volume or more of the resulting metallic glass is in an amorphous phase or nano-crystals of less than 100 nm in this art.
Compositions of amorphous alloys, known as metallic glasses, are being developed in the industry. Conventional production methods include the ones the inventors have disclosed. They are (1) the differential pressure casting technique (Japanese Kokai No. H8-109419), (2) the zone melting technique (Japanese Kokai No. H8-120363), and (3) the die casting technique (Japanese Kokai No. H8-199318). Yet there is another patent application which discloses a new composition and production method of an amorphous alloy. Japanese Kokai No. H9-323146 discloses a composition of 41.2 atomic % Zr, 13.8 atomic % Ti, 10.0 atomic % Ni, 12.5 atomic % Cu, and 22.5 atomic % Be, and the alloy melt is injected into a die-cast at 500 psi or greater.
Generally, a metallic glass material is formed by taking advantage of good viscous flow that exists in the supercooled liquid region of an amorphous alloy. Japanese Kokai No. H10-216920 and Japanese Kokai No. H10-249600 disclose a method in which a metallic glass material is heated to a temperature within the supercooled region, followed by press molding the metallic glass. Japanese domestic announcement Kohyo No. 8-508545 discloses a metallic glass of a composition expressed by the following chemical formula: (Zr1xe2x88x92xTix)a(Cu1xe2x88x92yNiy)bBec, wherein the composition is excellent in bending ductility and can be rolled to have ⅓ of the initial thickness.
However, a metallic glass having an alloy composition of Zr55Cu30Al10Ni5, for example, has a transition temperature (Tg) of 420xc2x0 C. and a crystallization temperature (Tx) of 500xc2x0 C. The metallic glass of this type is viscously fluid in the supercooled liquid region, which exists between the transition temperature and the crystallization temperature. Although this type can be formed well within the supercooled liquid region, the metallic glass product made by the conventional rapid cooling technique has a draft (reduction ratio) of only 40% at maximum when cold rolled.
There have been no reports teaching that a bulk metallic glass can possibly be cold rolled if the bulk metallic glass is made by such conventional casting techniques as melt forging; die casting; press molding of a melt injected into a mold; dual-roller solidification, or by the conventional water quenching technique. Moreover, the inventors"" experiments even confirmed that it is impossible to cold rolling a bulk metallic glass made by conventional technology.
Some amorphous alloys, having a fine crystalline structure consisting of nano-particles of less than 100 nm in a amorphous phase matrix, are known for their improved mechanical and chemical properties. For these alloys, a fine crystalline structure consisting of nano-particles is obtained by heating an amorphous alloy at a temperature below its crystallization temperature. (Japanese Kokai No. H7-188878; Japanese Kokai No H8-109454; Japanese Kokai No. H9-300063; and Japanese Kokai No. H10-218700.)
The inventors of the present invention have studied the quenching technique for making a metallic glass of excellent cold deformation properties, and confirmed that alloys of Zrxe2x80x94Tixe2x80x94Alxe2x80x94Cuxe2x80x94Ni system are excellent in their glass forming capability, heat stability, and mechanical properties. The inventors also found that the critical cooling rate for glassifying this alloy system was 10-100 K/s, at which a bulk metallic glass of 30 mm or less in diameter can be obtained by a variety of casting techniques. The alloy had an improved draft of 50% or more when cold rolled, and the cold rolled metallic glass sheet shows an excellent toughness. For example, the alloy glass could be cold rolled using regular rollers to obtain a very thin metallic glass sheet reflecting its ability to be reduced by 90% or more.
Nonetheless, such a thin metallic glass sheet made by a conventional casting technique had a drawback, in that its hardness deteriorated, and its tensile strength became poorer than that of an as-cast material as a draft increased. The process was so immature that it was impossible to produce a highly reliable material. To resolve the issue, the present invention intends to provide a bulk metallic glass, that is suited to cold working (e.g. cold rolling) because of its excellent draft (xe2x80x9ccold rolling reduction ratioxe2x80x9d for cold rolling) of 70% or more and its excellent mechanical properties. More specifically, the required mechanical properties after cold rolled are better elastic elongation and bending properties than those of the glass as cast. These properties can provide sheet materials or wire materials of various cross sections. The present invention also intends to provide the production method of the bulk metallic glass.
The inventors rigorously studied ways to make a bulk metallic glass of excellent ductility and having post cold rolling mechanical properties that is in a mono glassy phase and a mixture of glass phase and crystalline phase, or a mixture of glass phase and nanocrystalline phases (ultra fine crystals of 100 nm or less). The inventors, then, found that a novel process, being characterized by dispersion of nano-particles throughout an amorphous phase, can provide such a bulk metallic glass, which could not have been obtained by any of the conventional techniques, such as the rapid cooling, water quenching, melt forging, die casting, press casting of a melt in a mold, in addition to any of the related art that has been developed by the inventors such as the differential pressure casting technique, the zone melting technique, and the casting technique using metallic dies. The present invention is presented herein as a result of the quest.
The first aspect of the present invention is a highly-ductile nano-particle dispersed metallic glass; the bulk metallic glass is obtained by solidifying an alloy melt (hereafter referred to as the xe2x80x9cmelt press solidification techniquexe2x80x9d) of composition capable of being glassified between a cooled upper mold and a cooled lower mold by pressing and expanding, and nano-particles are dispersed in its amorphous phase, thereby obtaining metallic glass having nano-particles of a draft of 70% or more when cold rolled.
The second aspect of the present invention is a metallic glass with high elastic elongation and excellent bending properties consisting essentially of a single amorphous phase. The metallic glass is obtained by cold working to expand the highly-ductile nano-particle dispersed metallic glass until the nano-particles disappear therefrom.
The third aspect of the present invention is a process of producing highly-ductile nano-particle dispersed metallic glass; the highly-ductile nano-particle dispersed bulk metallic glass is produced by pressing to expand an alloy melt of a composition capable of being glassified between an upper mold and a lower mold. The upper mold and the lower mold are a pair of highly heat-conductive water-cooled casting molds, in which the alloy melt is solidified by a pressing to expand.
One of the preferable modes of the present invention is a process in which the upper and lower molds are positioned in relative proximity such that a melt can be pressed at 0.5-5 Kg/cm2 therebetween in the direction orthogonal to the expanded direction while the melt is being solidified.
Another preferable mode of the present invention is the process in which a water cooled copper mold is used. The copper mold is loaded with an alloy material of a composition capable of being glassified for forming a metallic glass and is melted by an arc melting technique.
The fourth aspect of the present invention is the process of producing a metallic glass of excellent elastic elongation and bending properties obtained by cold working to expand the highly-ductile nano-particle dispersed metallic glass, which is obtained by the melt press solidification technique. The cold working of this invention allows cold rolling of a highly-ductile nano-particle dispersed metallic glass with regular reduction rolls or roller dies, thereby producing sheet materials and wire materials of various cross sections readily.
None of such techniques as die casting, press forming of a melt injected into a mold, differential press molding (the related art of the inventor) could produce a bulk metallic glass dispersed with nano-particles in an amorphous phase is obtained and the amorphous phase providing a draft of 70% or more when cold rolled.
A method of producing a high strength metallic material with a uniform fine structure that is free of voids is disclosed in Japanese Kokai No. H8-168868. The structure is obtained by forging a melt having composition of Mg72Cu20Y8 under supercooled conditions. In this melt forging technique, a melt injected into a mold is pressed at 2,000 Kgf/cm2, which is two digits larger than that of the present invention. This technique is also unable to provide a metallic glass suited to cold working to expand.
The metallic glass material with dispersed nano-particles obtained by the melt press solidification technique of the present invention is characterized in that the metallic glass material with dispersed nano-particles has a smaller number of inner defects than the metallic glass material obtained by any of the conventional casting techniques such as melt forging, die casting, and differential casting, in addition to the water quenching technique. The metallic glass with dispersed nano-particles obtained by the melt press solidification technique is further characterized by nano-particles of about several nm-100 nm dispersed throughout the amorphous phase. As a result, the resulting product has improved plasticity, ductility, and mechanical properties.
Also, according to the melt press solidification technique of the present invention, the metallic glass material obtained by cold drawing the nano-particle dispersed metallic glass such as a cold rolling technique and the like has no nano-particles due to the disappearing of the nano particles by the mechanical alloying effect, thereby creating essentially a single amorphous phase. Compared with the material produced only by the casting process, having tensile strength of 1,700 MPa, elastic elongation rate of 2%, and bending strength of 2,000 MPa, the metallic glass material resulting from cold working of the nano-particle dispersed metallic glass material mentioned above is characterized, for example, by somewhat less tensile strength of 1,506 MPa, an improved elastic elongation rate of 2.8%, and a higher bending strength of 3,000 MPa.